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A Heterogeneous Nickel Catalyst for the Hydrogenolysis of Aryl Ethers without Arene Hydrogenation Alexey G. Sergeev, Jonathan D. Webb, and John F. Hartwig J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja3085912 • Publication Date (Web): 19 Nov 2012 Downloaded from http://pubs.acs.org on November 30, 2012
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Journal of the American Chemical Society
A Heterogeneous Nickel Catalyst for the Hydrogenolysis of Aryl Ethers without Arene Hydrogenation Alexey G. Sergeev, Jonathan D. Webb and John F. Hartwig* Department of Chemistry, University of California, Berkeley, Berkeley, California 94720-1460, United States Supporting Information Placeholder
ABSTRACT: A heterogeneous nickel catalyst for the selective hydrogenolysis of aryl ethers to arenes and alcohols generated without added dative ligand is described. The catalyst is formed in situ from the well-defined soluble nickel precursors Ni(COD)2 or Ni(CH2TMS)2(TMEDA) in the presence of a base additive, such as tBuONa. The catalyst selectively cleaves CAr-O bonds in aryl ether models of lignin without hydrogenation of aromatic rings, and it operates at loadings down to 0.25 mol% at 1 bar of H2 pressure. The selectivity of this catalyst for electronically varied aryl ethers differs from that of the homogeneous catalyst reported previously, implying that the two catalysts are distinct from each other.
Catalytic hydrogenolysis of the CAr-O bond in aromatic ethers is a critical process for the conversion of the lignin component of plant biomass into aromatic hydrocarbons, feedstocks for the production of biofuels, and chemicals.1-4 Typically, the hydrogenolysis of CAr-O bonds is conducted over heterogeneous catalysts that require high temperatures (>250 ºC) and pressures of hydrogen (>30 bar). These conditions lead to concomitant reduction of aromatic rings.1,5-7 The poor chemoselectivity of heterogeneous catalysts for hydrogenolysis over hydrogenation wastes hydrogen and results in low yields of arene products. Therefore, the identification of catalysts that cleave CAr-O bonds selectively would be a significant advance toward addressing the challenge of converting lignocellulose biomass into simple arenes.3 To address the problem of selective reduction of CAr-O bonds in aryl ethers, several groups have developed catalytic systems that are based on soluble nickel complexes and hydride donors, such as silanes8-10 and aluminohydrides.10 Reactions with hydrogen would be more practical, and we recently reported a nickel-Nheterocyclic carbene catalyst for the selective reduction of the CAr-O bond in aryl ethers with hydrogen at 1 bar pressure.10 The catalyst converted aryl ethers into arenes and alcohols in high yields without arene hydrogenation. Despite these advantages, the process required high cat-
alyst loading (typically 20 mol % of Ni) and the SIPr carbene ligand. Here we report a highly active heterogeneous and ligandless nickel catalyst11 for selective hydrogenolysis of diaryl, benzyl aryl, and benzyl alkyl ethers at much lower loadings (down to 0.25%) than the SIPr-Ni catalyst without carbenes or phosphines as auxiliary ligands. In contrast to most known heterogeneous systems, the catalyst operates at low hydrogen pressure (1 bar) and does not catalyze the hydrogenation of arenes. The regioselectivity of this system for cleavage of two different types of C-O bonds is orthogonal to that of the previously reported homogeneous nickel-carbene catalyst.10 The ligandless nickel catalyst reported here was discovered when studying the effects of ligands on the nickel-catalyzed hydrogenolysis of aryl ethers.10 Initial studies showed that Ni(COD)2 without added ligand was less reactive than the combination of Ni(COD)2 and SIPr·HCl for the hydrogenolysis of diphenyl ether (eq 1).
However, we have now found that this ligandless system is more active for hydrogenolysis of the types of electron-rich diaryl ethers found in lignin1,12,13 than is the SIPr-Ni catalyst. Figure 1A provides data on the relative rates for reaction of electronically varied diaryl ethers in the presence of the two types of catalysts. Only 75% conversion of the di-o-anisyl ether to arene and aryl alcohol occurred in the presence of the Ni-SIPr system under the conditions that led to full conversion of the same ether in the presence of the ligandless system. In contrast, conditions that led to full conversion of the trifluoromethyl-substituted ether in the presence of the Ni-SIPr system occurred to only 41% conversion in the presence of the ligandless system and formed products from the reduction of both C-O and C-F bonds. To test whether this trend in reactivity also applies to the cleavage of two different CAr-O bonds within the
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same ether, we conducted the hydrogenolysis of unsymmetrical diaryl ethers. As shown in Figure 1B, the ligandless nickel catalyst cleaved the CAr-O bond adjacent to the most electron-rich arene ring of 4-hydroxy diphenyl ether to form 2 equiv of phenol as the sole product. In contrast, the Ni-SIPr catalyst preferentially cleaved the CAr-O bond adjacent to the more electrondeficient ring to form predominantly hydroquinone (67%) and benzene (48%). These results on both the intermolecular and intramolecular competition experiments clearly indicate that the catalytic species generated without added ligand is distinct from the species generated with added SIPr ligand. Figure 1. Differences in reactivity and selectivity between the ligandless and carbene-‐ligated nickel catalysts. A. Orthogonal reactivity O R1
OMe
HO +
20 mol% "Ni" + H2 t BuONa, m-xylene, (1 bar) 120 °C, 16 h R1 R2
R2
Relative reactivities of diaryl ethers (reaction time 16 h) OMe O O O CF3
100%
62%
41%
conversion with the ligandless Nia 100%b
75%
100%c
conversion with the carbene Ni
B. Orthogonal selectivity Ligandless Ni O HO
Carbene Ni H2 1 bar m-xylene, 120 °C, 16h
HO 20% Ni(COD)2a 2 tBuONa Conversion: 100%
97%
20% Ni(COD)2, 40% SIPr·HCl HO 67% tBuONa HO Conversion: 67%
loadings were not minimized. at 100 oC; c100% at 100 oC
base occurred to completion and formed benzene and phenol in nearly quantitative yields (Table 1, Entry 4).14 Table 1. Effect of base and nickel precursor on hydrogenolysis of diphenyl ether catalyzed by the ligandless nickel.a
Entry
t
BuONa, equiv
Conv., %
GC Yields A, %
B, %
C, %
D, %
E, %
1 0 72b 58 2 12 53 6c 2 0.2 19 d 18 8 0 8 0 3 1 84d 84 77 0 5 0 4 2.5 100 99 97 0
